1. Field of the Invention
The present invention relates generally to a driving circuit; particularly, the present invention relates to a light-emitting diode (LED) driving circuit capable of decreasing the cost and improving efficiency.
2. Description of the Related Art
For enhancing power conversion efficiency of driving circuit, the conventional LED driving circuit utilizes a switching power converter to improve power conversion efficiency. For instance, as shown in FIG. 1, a rectifier 2 converts AC (alternating current) power into full-wave rectification voltages and transmits the full-wave rectification voltages to a transformer 3. It is noted that an output end of the rectifier 2 is coupled with an active switch SW1 and a storing capacitor C1, and the other end of the transformer 3 is coupled with a diode 5 and an output capacitor C2.
In addition, a controller 4 controls a driven state of the active switch SW1 and connects the output capacitor C2 through a feedback module 6. In practical applications, the feedback module 6 is connected to the output voltage Vout and can retrieve a feedback signal (voltages, currents, or duty) of the output voltage Vout. It is noted that the controller 4 controls a pulse width of a control signal of the active switch SW1 according to a difference between the feedback signal of the output voltage Vout and a reference voltage Vref so that the output voltage Vout and current is maintained at a constant. In other words, the driving circuit of FIG. 1 utilizes the controller 4 instead of a resistive regulator to maintain stability of the output voltage so as to increase power conversion efficiency.
However, the driving circuit includes high-frequency electronic components, such as the storing capacitor C1 and the output capacitor C2. In other words, when AC power is transmitted into the storing capacitor C1 and the output capacitor C2, the driving circuit generates visual work, so that the driving circuit cannot achieve the high power factor. As shown in FIG. 1, R&D (research and development) people try to utilize a power-factor-adjusting module 7 to improve the low power factor phenomena. However, the power-factor-adjusting module 7 results in a larger size driving circuit and additional material cost and is hard to be used in small sized illuminating products.
In addition, another conventional LED driving circuit utilizes AC/DC power converter to generate the output voltage, and the output voltages drive the LEDs to generate light. In practical applications, the LED driving circuit includes a power supply module, wherein the power supply module controls current of the circuit, so that the current amplitude crossing over the LEDs remains constant, and the brightness of the LEDs maintains constant.
Particularly, the conventional LED driving circuit further includes a plurality of switches and a plurality of comparators corresponding to the switches, wherein the switches are respectively coupled with corresponding comparators and LEDs. In addition, each comparator has a constant voltage and determines whether to transmit activating control signal to the switches according to the relation between the constant voltage and the input voltage. In practical applications, the conventional LED driving circuit controls the light-emitting result of driving the LEDs according to the activating condition of the switches. In other words, if more switches are driven to be activated, more LEDs generate light. However, in the circuit, because each switch corresponds to one comparator, the circuit requires lots of comparators, resulting in a complicated driving circuit and the increase in production cost.
For the above reasons, it is an object how to design an LED driving circuit for enhancing efficiency and decreasing the cost.